In zoom lenses mainly used in consumer video cameras, a so-called four group inner focus zoom system is the main stream, which has a four group configuration in which refracting power arrangement is positive, negative, positive, and positive in order from an object side, wherein a first lens group and a third lens group are stationary, and variable power is mainly performed by shifting a second lens group in an optical axis direction, and correction for image position fluctuations and focusing are performed by shifting a fourth lens group in the optical axis direction. As the configuration of the zoom lens related to this system, there have been proposed many different types, such as those described in Japanese Patent Application Laid-Open Nos. Hei 3-33710 and Hei 4-153615.
In these lens configurations, the lens configurations of the first lens group and the second lens group employ a very similar lens type, so that the angle of view of a picked-up image diagonal at a wide angle end is about 60 degrees at the utmost. For example, one described in Japanese Patent Application Laid-Open No. 2000-28922 attempts to achieve miniaturization of a front lens diameter by bringing an image side principal point of the first lens group closer to the surface closest to an image side of the first lens group, but fails to achieve widening of the angle of view at a wide angle end to not less than 60 degrees, thus failing to accomplish compatibility between widening of angle and miniaturization of the front lens diameter.
As an example of attempt to achieve sufficient widening of angle, there is known one described in Japanese Patent Application Laid-Open No. Hei 5-72475, which has developed the first lens group from a three-lens configuration into a five-lens configuration, on the basis of Japanese Patent Application Laid-Open No. Hei 3-33710.
There has also been proposed to correct a distortion that varies depending on zooming (variable power) by an electric signal processing technique on an image pickup apparatus side. For example, Japanese Patent Application Laid-Open No. Hei 6-165024 is known.
In the zoom lens described in Japanese Patent Application Laid-Open No. Hei 5-72475, based on the lens type shown in Japanese Patent Application Laid-Open No. Hei 3-33710, the inclination of a principal ray to the third and later lenses of the first lens group is reduced to permit correction for various aberrations by disposing a concave lens and a convex lens having large air spacing therebetween on the object side of the first lens group of the three-lens configuration, in order to add a configuration close to an afocal system, such as a wide conversion lens.
It is however necessary to dispose the added two lenses with large air spacing, in order to correct properly in balance the distortion of a wide angle end that tends to increase due to widening of angle and meridional curvature of field, so an increase in front lens diameter is unavoidable. Moreover, since the zoom lens is the invention made only for the purpose of widening of angle of the lens configuration of Japanese Patent Application Laid-Open No. Hei 3-33710, it is realized by strictly regulating the lens configuration of the first lens group through the fourth lens group. With regard to specifications such as zoom ratio and F-number, front lens diameter, total length, back focus, etc., an optimum lens configuration for the intended purpose is not always obtainable.
The present invention has for its subject to provide a wide-angle zoom lens most suitable for various specifications, which enables such widening of angle that the angle of view at a wide angle end is not less than 60 degrees, by making a first lens group into a five-lens configuration different from Japanese Patent Application Laid-Open No. Hei 5-72475, in combination with many different variations of so-called four-group inner focus system zoom lens, and in which increase in front lens diameter is minimized to achieve the harmonization between widening of angle and miniaturization of front lens diameter, and many different types of variations of conventional types are applied to a third lens group and a fourth lens group.
Further miniaturization is also enabled in the following manners that distortion, the correction for which inevitably becomes difficult by achieving the harmonization between widening of angle and miniaturization of front lens diameter, is corrected by a video signal processing, and that the ratio of the angle of view of a wide angle end to that of a telephoto end, obtainable from an image surface after distortion correction, is redefined as a zoom ratio, thereby reducing paraxial focal length ratio (general definition of zoom ratio). The present invention has for its subject to provide an image pickup apparatus that permits miniaturization for a zoom ratio required, by actively and largely causing negative distortion at a wide angle end and positive distortion at a telephoto end, so that the change in the angle of view after distortion correction is sufficiently greater for the change in paraxial focal length.